1. Field of the Invention
The present application relates to a wholly aromatic liquid crystalline polyester and a method for preparing the same. In particular, the present invention relates to liquid-crystalline polyester which exhibits high heat stability, excellent moldability and flowability as well as excellent mechanical properties.
2. Related Art
Thermotropic liquid-crystalline polyester (which is called as “liquid-crystalline polyester” or “LCP” hereinafter) has good properties including heat resistance, mechanical properties such as rigidity, chemical resistance and dimensional accuracy and is used not only for molded articles but also for a variety of products such as fibers and films. Particularly, personal computers and mobile phones employ highly integrated devices and the art wishes to use downsized, thinner and smaller parts for them. In the information and telecommunication fields, very thin parts, as thin as 0.5 mm or less of the thickness, are sometimes required. Based on the excellent molding properties of LCPs including good flowability and less flash development compared to the other thermoplastic resins, consumption of LCPs has been increasing.
Recently, LCPs are employed for a variety of uses wherein the resins are processed at a high-temperature such as solder-welding and infrared reflow-welding. However, the LCPs produced by polymerization method, such as slurry polymerization or molten acidlysis, do not exhibit sufficient heat resistance.
In order to obtain a LCP with an excellent heat resistance, methods using solid phase polymerization step have been proposed. For example, U.S. Pat. No. 6,582,625, Japanese Patent Application Laid Open No. 2000-248056 and Japanese Patent Application Laid Open No. Hei. 11-246654 (the cited references are herein incorporated by reference) disclose methods for producing LCPs, which comprise the steps of preparing powdery or pellet-formed prepolymer with a low polymerization degree and heating said prepolymer in solid phase to give polymers with high polymerization degree. In general, thus obtained LCPs have high melting point besides the high heat resistance and therefore, need to be processed with relatively high temperatures.
Japanese Patent Examined Publication No. H03-55489 proposed LCP obtained by copolymerizing 4-hydroxybenzoic acid, 2,6-naphthalene dicarboxylic acid and hydroquinone with a small amount of 6-hydroxy-2-naphthoic acid. According to the patent, thus obtained LCP will exhibit good mechanical and thermal properties and provide good moldability.
However, what the Japanese Patent Examined Publication No. H03-55489 disclosed is the LCP obtained by melt polymerization of the monomers and is silent about LCPs which is subjected to heat treatment in a substantially solid state.